Wideband code division multiple access (W-CDMA) schemes, which are third generation (3G) radio access schemes of cellular mobile communication, have been standardized in 3rd Generation Partnership Project (hereinafter, referred to as 3GPP), and cellular mobile communication services by the same scheme have been initiated. In 3GPP, the evolution of 3G (hereinafter, referred to as evolved universal terrestrial radio access (EUTRA)) and the evolution of a 3G network (hereinafter, referred to as evolved universal terrestrial radio access network (EUTRAN)) have been studied.
An orthogonal frequency division multiplexing (hereinafter, referred to as OFDM) scheme of multicarrier transmission is used in a downlink of transmission from a base station device of EUTRA to a mobile station device. A single-carrier communication scheme of a discrete Fourier transform (DFT)-spread OFDM scheme of single-carrier transmission is used in an uplink of transmission from a mobile station device of EUTRA to a base station device.
In 3GPP, a 4th generation (4G) radio access scheme of cellular mobile communication (advanced EUTRA (hereinafter, referred to as “A-EUTRA”)) and a 4G network (advanced EUTRAN) has started to be studied.
In A-EUTRA, a study has been made to deal with a frequency band, which is wider than that of EUTRA, and to secure compatibility with EUTRA. A mobile station device corresponding to EUTRA, which communicates with a base station using part (hereinafter, referred to as a “subband”) of a frequency band of the base station device, and a mobile station device corresponding to A-EUTRA, which communicates with a base station device using one or more subbands of the base station device, have been proposed. That is, the base station device, which controls transmission/reception of the mobile station device, performs transmission/reception to/from the mobile station device corresponding to EUTRA using any one subband, and perform is transmission/reception to/from the mobile station device corresponding to A-EUTRA using one or more subbands in response to capability of the mobile station device, which performs transmission/reception. That is, the base station device uses all subbands by allocating several resources (a resource block) of a subband to the mobile station device corresponding to EUTRA in each subband and allocating several separate resources, which are not yet allocated to the mobile station device corresponding to EUTRA, to the mobile station device corresponding to A-EUTRA. However, the number of resources to be allocated is changed in response to capability of the mobile station device corresponding to A-EUTRA since the number of subbands capable of being simultaneously transmitted and received is different even in the mobile station device corresponding to A-EUTRA.
A layered OFDM scheme, which uses OFDM of multicarrier transmission in a downlink of transmission from a base station device to a mobile station device in A-EUTRA and performs communication using a plurality of frequency bands, has been proposed (see Non-Patent Document 1).
FIG. 15 is a diagram showing a schematic configuration of a downlink radio frame in EUTRA. In FIG. 15, the horizontal axis represents a frequency domain and the vertical axis represents a time domain. The downlink radio frame is a unit of radio resource allocation or the like, and includes physical resource block (hereinafter, referred to as “PRB”) pairs including a frequency band and a time band having predetermined widths. One PRB pair includes 2 PRBs continuous in the time domain.
One PRB includes 12 subcarriers in the frequency domain and includes 7 OFDM symbols in the time domain. A system bandwidth is a communication bandwidth of the base station device. In the time domain, a slot includes 7 OFDM symbols, a subframe includes 2 slots, and a radio frame includes 10 subframes. A unit including 1 subcarrier and 1 OFDM symbol is referred to as a resource element. In a downlink radio frame, a plurality of PRBs are arranged in response to the system bandwidth.
In each subframe, at least a downlink shared data channel used in transmission of information data and a downlink control channel used in transmission of control data are arranged. Although not shown in FIG. 15, downlink pilot channels used in channel estimation of the downlink shared data channel and the downlink control channel are distributed and arranged in a plurality of resource elements. The case where the downlink control channel is arranged in first, second, and third OFDM symbols of the subframe, and the downlink shared data channel is arranged in other OFDM symbols is shown in FIG. 15, but the OFDM symbols in which the downlink control channel is arranged may vary in a subframe unit.
Although not shown in FIG. 15, a control format indicator channel indicating the number of OFDM symbols constituting the downlink control channel is arranged in the first OFDM symbol. The downlink control channel may be arranged in only the first OFDM symbol or may be arranged in the first and second OFDM symbols. In the same OFDM symbol, the downlink control channel and the downlink shared data channel are not arranged together. In the downlink control channel, a mobile station ID, radio resource allocation information of the downlink shared data channel, multi-antenna-related information, a modulation scheme, a coding rate, a retransmission parameter, and the like are arranged.
The downlink control channel is constituted by one or more control channel elements (CCEs). The number of CCEs depends upon the system bandwidth, the number of OFDM symbols constituting the downlink control channel, and the number of downlink pilot channels corresponding to the number of transmission antennas of the base station device used in communication. The CCE is constituted by a plurality of resource elements.
FIG. 16 is a diagram illustrating a logical relationship between the CCEs and the downlink control channel in EUTRA. Here, CCE n indicates a CCE of CCE index n. The CCE index is a CCE identification number.
The downlink control channel is constituted by an aggregation including one or more CCEs. The number of CCEs constituting the aggregation is hereinafter referred to as a “CCE aggregation number.” The CCE aggregation number constituting the downlink control channel is determined in response to a coding rate and an amount of control data. An aggregation including n CCEs is hereinafter referred to as “CCE aggregation n.” For example, the downlink control channel is constituted by 1 CCE (CCE aggregation 1), the downlink control channel is constituted by 2 CCEs (CCE aggregation 2), the downlink control channel is constituted by 4 CCEs (CCE aggregation 4), or the downlink control channel is constituted by 8 CCEs (CCE aggregation 8).
The CCE is constituted by a plurality of resource element groups (also referred to as mini-CCEs). FIG. 17 is a diagram illustrating an arrangement example of resource element groups in a downlink subframe by EUTRA. Here, the case where the downlink control channel is constituted by first to third OFDM symbols and downlink pilot channels of 2 transmission antennas (transmission antenna 1 and transmission antenna 2) are arranged is shown. In FIG. 17, the horizontal axis represents a frequency domain and the vertical axis represents a time domain. In the arrangement example of FIG. 17, 1 resource element group is constituted by 4 resource elements and is constituted by adjacent resource elements of the frequency domain.
If resource elements are denoted by the same reference numeral as that of the downlink control channel in FIG. 17, it indicates that the resource elements belong to the same resource element group. A resource element group is configured by skipping a resource element R1 (a downlink pilot channel to be transmitted from transmission antenna 1) and a resource element R2 (a downlink pilot channel to be transmitted from transmission antenna 2) in which the downlink pilot channels are arranged.
In FIG. 17, numbering (reference numeral “1”) is performed from a resource element group of a first OFDM symbol having a lowest frequency. Next, a resource element group of a second OFDM symbol having a lowest frequency is numbered (reference numeral “2”). Next, a resource element group of a third OFDM symbol having a lowest frequency is numbered (reference numeral “3”).
Next, a resource element group is numbered (reference numeral “4”) adjacent in the frequency axis to the resource element group (reference numeral “2”) in which the second OFDM symbol in which no downlink pilot channel is arranged is numbered. Next, a resource element group is numbered (reference numeral “5”) adjacent in the frequency axis to the resource element group (reference numeral “3”) in which the third OFDM symbol in which no downlink pilot channel is arranged is numbered.
Next, a resource element group is numbered (reference numeral “6”) adjacent in the frequency axis to the resource element group (reference numeral “1”) in which the first OFDM symbol is numbered. Next, a resource element group is numbered (reference numeral “7”) adjacent in the frequency axis to the resource element group (reference numeral “2”) in which the second OFDM symbol is numbered. Next, a resource element group is numbered (reference numeral “8”) adjacent in the frequency axis to the resource element group (reference numeral “3”) in which the third OFDM symbol is numbered. Likewise, resource element groups of subsequent PRB pairs are also numbered.
The CCE is constituted by a plurality of resource element groups constituted as shown in FIG. 17. For example, 1 CCE is constituted by 9 different resource element groups distributed in the frequency domain and the time domain. Specifically, all resource element groups numbered as shown in FIG. 17 in the entire system bandwidth are interleaved using a block interleaver in a resource element group unit, and 1 CCE is constituted by 9 resource element groups whose numbers are continuous after interleaving.
The mobile station device demodulates and decodes a reception signal under assumption of a plurality of downlink control channels capable of being allocated to its own mobile station device for CCEs received in each subframe, and performs a cyclic redundancy check (hereinafter, referred to as “CRC”) of checking whether or not it is a downlink control channel allocated to its own mobile station device using a CRC code added to the downlink control channel. Specifically, the base station device generates the CRC code from control data using a predetermined generating polynomial, adds information (CRC masked by UE ID) obtained by an exclusive OR operation of the generated CRC code and a mobile station ID of the mobile station device to which a downlink control channel is allocated to the downlink control channel, multiplexes the downlink control channel into a CCE, and transmits the CCE. By performing the inverse processing of the above-described operation, the mobile station device receiving the CCE from the base station device performs error detection and also detects whether or not a downlink control channel addressed to its own mobile station device is multiplexed and transmitted.
For example, in the case of a downlink control channel as shown in FIG. 16, a reception signal is demodulated, decoded, and CRC-checked for a total of 15 CCE combinations of 8 CCE aggregations 1, 4 CCE aggregations 2, 2 CCE aggregations 4, and 1 CCE aggregation 8 in terms of CCEs 1 to 8 under the assumption that the downlink control channel is multiplexed and transmitted. This processing is referred to as blind decoding of the downlink control channel, and the number of times of blind decoding is increased with an increase of the number of possible CCEs.
Here, a modulation scheme of the downlink control channel is fixed and several candidates for a coding rate are set for each CCE aggregation number. Accordingly, when the blind decoding is performed, decoding and a CRC check are performed at each candidate coding rate corresponding to a CCE aggregation number for each CCE combination. That is, if the number of candidate coding rates corresponding to a CCE aggregation number of a certain CCE combination is 2, decoding and a CRC check are performed for the CCE combination using each of the 2 coding rates, so that decoding and a CRC check are performed in two ways for the CCE combination. At this time, the coding rate varies with an amount of control data to be transmitted on the downlink control channel. Since the coding rate is determined by the CCE aggregation number if the amount of control data of the downlink control channel is fixed, decoding and a CRC check are performed in one way for each CCE combination. When the system bandwidth is wide, the number of CCEs is increased, the number of times of blind decoding of the downlink control channel is increased, and the processing load of the mobile station device is increased.
Thus, a method of reducing the number of times of blind decoding is used. Each mobile station device sets a CCE for which the downlink control channel is decoded. Specifically, the mobile station device sets a CCE number (hereinafter, referred to as a “starting point index”) from which the downlink control channel starts to be decoded for each CCE aggregation number by a hash function having an input of a mobile station ID. The mobile station device decodes the downlink control channel using a plurality of CCEs from the set starting point index (hereinafter, a space including a plurality of CCEs for which the mobile station device determines the downlink control channel is referred to as a “mobile station-specific search space (UE-specific search space)”).
The base station device recognizes a mobile station ID of the mobile station device to which the downlink control channel is allocated, multiplexes the downlink control channel including control data specific to the mobile station device into a CCE within a mobile station-specific space determined in response to the mobile station ID, and transmits the CCE to the mobile station device. As described above, a method of reducing the number of times of decoding the downlink control channel in the mobile station device is used by limiting the CCE for which the mobile station device decodes the downlink control channel.
In A-EUTRA, a study has been made to deal with a frequency band, which is wider than that of EUTRA, and to secure compatibility with EUTRA. For example, a wireless communication system including a plurality of frequency bands (a plurality of subbands) by designating a frequency bandwidth of EUTRA as one unit (subband) has been studied.
In A-EUTRA, which is a wireless communication system including a plurality of subbands, a plurality of methods for radio resource allocation information indicating radio resource allocation of a downlink shared data channel included in a downlink control channel have been studied (see Non-Patent Document 2).
For example, a method using the same radio resource allocation information as that of EUTRA has been studied (hereinafter, this method is referred to as “radio resource allocation method 1”). In radio resource allocation method 1, radio resource allocation information included in the downlink control channel corresponds to only within a subband in which the downlink control channel is arranged and indicates which PRB pair is allocated to the downlink shared data channel of the mobile station device to which the downlink control channel is allocated within a subband in which the downlink control channel is arranged. Radio resource allocation method 1 can reduce design and operation test loads of the mobile station device since the same radio resource allocation information as that of EUTRA is used.
Another method of adding subband information to radio resource allocation information of EUTRA has been studied (hereinafter, this method is referred to as “radio resource allocation method 2”). In radio resource allocation method 2, radio resource allocation information included in the downlink control channel is constituted by information indicating which PRB pair within a single subband is allocated to the downlink shared data channel of the mobile station device to which the downlink control channel is allocated and information indicating which subband corresponds to the PRB pair.
Since radio resource allocation method 2 requires a new control data format including subband information as compared to radio resource allocation method 1, design and operation test loads of the mobile station device are slightly increased. However, it is possible to increase a degree of freedom of scheduling of the base station device since a PRB pair of the downlink shared data channel indicated by the radio resource allocation information of the downlink control channel is not limited to a subband in which the downlink control channel is arranged.
Non-Patent Document 1: 3GPP TSG RAN1 #53, Kansas City, USA, 5-9 May, 2008, R1-081948 “Proposals for LTE-Advanced Technologies”
Non-Patent Document 2: 3GPP TSG RAN1 #53 bis, Warsaw, Poland, Jun. 30-Jul. 4, 2008, R1-082468 “Carrier aggregation in LTE-Advanced”